Disable Back Button - This is very annoying and I hate site's that do this. If you have to trap people from exiting your site you have a problem. The hits you gain from this practice is not worth the visitors you piss you off by doing this.

Sound - Sound from a website is very annoying when its unexpected. A lot of pages are starting to put movies and songs on their main page. This trend defitinely does not bring in more visitors but it sure does lose visitors. Just think of the person viewing a page at work and then hears music - right away they close the page never to go back to the site again.